Red Dwarf Stars and SETI

byPaul GilsteronNovember 29, 2005

M-class red dwarfs have never figured prominently in the SETI search. The reason for this is apparent: such stars, of which Proxima Centauri, Earth’s nearest stellar neighbor, is one, are flare stars. The intense radiation from solar flares should cleanse a planetary surface of life, especially given the close proximity of such a planet to its star. Remember, the habitable zone around a red dwarf is going to occur well inside the orbit of Mercury.

And there’s a second reason. By virtue of having to orbit the host star so tightly, a planet around a red dwarf is going to be tidally locked. One side would be baked, the other frozen, which makes the odds on liquid water look slim. But assumptions are made to be questioned, which is why work at Ames Research Center in the late 1990s remains so interesting. One implication of the Ames work, for example, is that there are conceivable weather patterns that could circulate heat to the dark side of a tidally locked world, keeping it warm enough to prevent its atmosphere from freezing out.

SETI pioneer Jill Tarter is well aware of such work, saying in a recent article in Astrobiology Magazine:

“If you put a little bit of greenhouse gas into an atmosphere, the circulations can keep that atmosphere at a reasonable temperature and you can dissipate the heat from the star-facing side and bring it around to the farside. And, perhaps, end up with a habitable world.”

That’s a big ‘perhaps,’ but it ties in with other recent work that shows that most of a red dwarf’s flare activity occurs early in its life cycle (this is why the recent observation of a major flare from Barnard’s Star is so unusual, given the star’s age). A quieting flare environment might allow life to survive. And because seven out of ten stars in our galaxy are red dwarfs like Proxima, we don’t want to ignore possible interesting targets as we focus on what seem the more likely F, G and K class stars.

As the Allen Telescope Array comes online, its 350 antennae will handle traditional radio astronomy as well as SETI, and with careful adjustment, astronomers will be able to form as many as eight virtual antennae, each pointing to a different star. An efficient SETI search will be one that allows the largest number of target stars in the array’s field of ‘view,’ and that means adding M-class dwarfs to increase the target list enormously. From the article:

“It’s not the star that I’m interested in,” Tarter says. “It’s the techno-signature from the inhabitants on a planet around the star. I don’t ever have to see the star, as long as I know that it’s in that direction. I don’t ever have to see the planet. But if I can see their radio transmitter – bingo! – I’ve gotten there. I’ve found a habitable world.”

Last July’s red dwarf workshop, chaired by Tarter, found no ‘showstoppers’ that would preclude adding red dwarfs to the target list. Findings from the conference will eventually be published. Until then, Ken Croswell’s story “Red, Willing and Able,” in New Scientist 169 (January, 2001, pp. 28-31) delivers a concise summary of the Ames work. See also Manoj Joshi, Robert Haberle, and R. Reynolds, “Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability,” Icarus 129 (1997), pages 450–65.

Comments on this entry are closed.

Jim StehrApril 26, 2007, 18:32

The evolution of life intelligent enough to have this conversation requires billions of years of stellar stability. If so, could a red dwarf qualify?

If you simulate the colour you get from Rayleigh scattering (responsible for Earth’s sky), you get either very pale bluish-gray for hotter red dwarfs, or yellowish-pink for cooler ones (the dividing line is somewhere around 3300 K).

However, this guy has simulated Mie scattering as well (and produced some images), and gets a greenish sky (!). Not sure if I believe that result, but I don’t know enough about modelling atmospheric scattering to check it.

Plus, if the atmosphere is thicker than Earth’s, multiple scattering becomes important, which changes things again.

That’s fascinating, Andy. Thanks, and thank you too, Peter, for a terrific question. Visualizing such places is a real exercise for the imagination. Not sure I believe the green sky result either, but it sure is pretty.

Well on Earth the sky is blue because of Rayleigh scattering of light, which increases with the inverse fourth-power of the wavelength – thus shorter blue wavelengths get scattered more than the others. The relative proportions are different in red dwarf spectra, but it is not a straightforward calculation as the way eyes respond to colour is quite complex. I suspect we would not notice the difference, but the sky might have a bit more yellow and seem kind of greenish.

Though we call them “red dwarfs” even the coldest is over about 2300 K in temperature and to the human eye anything over about 2000 K looks “white”, thus all the astroart that has garish red stars are taking a bit of artistic license. Even high mass “brown dwarfs” will look yellow-white. Below about 1500 K is when they start looking more red.

Even the planet Venus would glow deep cherry red in patches as its surface is over 750 K in places. Above about 900 K its clouds would disperse and the planet would glow dully through a thick layer of atmosphere – about 1000 bar would be enough. There probably isn’t enough carbon dioxide locked up as carbonate for that to happen, but it will eventually as the Sun brightens over the next 5 billion years.

Adam: the green colour doesn’t come from Rayleigh scattering: I modelled the colours from Rayleigh scattering by integrating the expected spectrum to get the colours in XYZ space, then transformed from XYZ to RGB (note this is not a unique transformation – you have to choose the whitepoint and the colour axes). For stars above 3300 K, my code gives a blue colour, starting from whitish but getting progressively deeper as the star temperature increases, going downwards from 3300 K gives progressively more orangey shades.

I was computing off the top of my head. Our eyes don’t see a collection of colours as strictly dominated by one particular band – think of the many different ways to make “white” – so I was guessing more yellow, less blue gives “green”. But thanks for the insight into your modelling – which is pretty cool.

Eyes react differently to the colours as well, complicating matters somewhat. My eyes see all sorts of spurious colours because I have some deficiency in sensitivity to colour – and I still don’t know what, as not even Air-Force examiners nor regular Optometrists can tell me. I just know I see things different. I can look into a “grey” sky and see either “pink” or “green”, yet I can look at light paint tints and only see a uniform grey. Weird.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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